Synthesis and structure elucidation of pyrimidobenzimidazoles and fused derivatives III [1,2].: Decahydropyrimido[1,2-a]benzimidazol-2-oles and octahydropyrimido[1,2-a]benzimidazoles

The reaction of racemic trans-hexahydrobenzimidazol-2-amine with three vinylogous ketones under mild conditions was studied. Only in the case of 4-phenyl-3-buten-2-one cycloaddition products could be isolated. According to NMR spectroscopy they consist of mixtures of two of eight possible diastereomers: rac-2 beta -methyl-4 beta -phenyl-trans-5a alpha- and trans-5a beta -decahydropyrimido [1,2-a]benzimidazol-2 alpha -ole. Reaction of the amine with the butenone at higher temperature and with, 2-methyl-1-phenyl-1-penten-3-one, and 4'-chlorochalcone afforded mixtures of two diastereomers each, which turned out as rac-4 alpha -phenyl-trans-5a beta- and trans-5a alpha -octahydropyrimido[1,2-a]benzimidazoles. Complete structural and stereochemical assignments of the title compounds and their hydrochlorides were established by NMR spectroscopic investigations. The results showed that all investigated cyclization reactions proceeded regioselectively with equal orientation of the components, but not diastereoselectively. Variation of the reaction conditions did influence neither regionor diastereoselectivity.